The present invention relates to a thin film field effect transistor device and to a display panel made using such devices. Thin film field effect transistors are now well known in the art, and typically use cadmium sulfide or cadmium selenide as the semiconducting material. It has been the practice to utilize gold, aluminum and indium-gold as the source and drain contacts of such devices. Double gated field effect transistors are also known in the art both for silicon devices and for thin film transistors. A double gated thin film transistor is shown in U.S. Pat. No. 3,500,142.
One of the prime areas for use of thin film transistors of the present invention are in large area flat panel display devices. In such devices an array of thin film transistors is disposed upon a substrate and used to control and drive an individual display medium associated with a specific unit display cell which is repeated across the entire area of the display panel. Such a device is seen in U.S. Pat. No. 3,840,695 in which a thin film transistor array is integrated with a liquid crystal display panel. In such thin film transistor control and drive flat panel displays, the array of display elements is interwoven together through thin film transistors via horizontal and vertical bus bars. The gates of one row of thin film transistors are connected to one horizontal bus bar and are electrically insulated from vertical source bus bars. Individual display elements can be addressed through the source bus bar only when a positive bias is applied to the gate of its corresponding thin film transistor and turns the transistor device on. In order to display information on the panel, electrical signals applied to both the gate and source bus bars must be free of distortion. If there are electrical shorts between the gates and sources, severe cross-talking between different display elements will occur and will result in smeared pictures. Other display mediums besides liquid crystals are suggested for use in such display panels, including electroluminescent phosphor display medium. The electroluminescent medium presents a more complex problem than the liquid crystal display medium because it requires a much higher operational voltage to produce light output. A significant problem with prior art thin film transistor devices has been that with the heretofore utilized source and drain contact materials the devices have been unable to withstand large voltages across the source and drain. Such prior art devices could thus not be used to effectively drive an electroluminescent display panel.
An important transistor operational parameter for transistor control flat panel display devices is that the devices have a high transconductance. A high transconductance means that the device has a high on-off ratio which is required because of the addressing schemes in which display information must be stored on a line of display elements and held at a fairly constant level until their refresh frame occurs. In the preferred unit cell control circuit for a flat panel display device a logic or switching transistor has its gate connected to a synchronizing signal bus bar which turns on the logic or switching transistor and permits display information to be addressed through the transistor to a storage capacitor. The logic or switching transistor is thereafter turned off and remains in an off state with the voltage on the capacitor being applied to the gate of a control transistor which actually controls power to the electro luminescent display medium. It is very important that the voltage on the storage capacitor remain relatively constant until the refresh frame. A further problem with prior art thin film transistors is the trapping of charges which can result in drift in the operational characteristic of the device. This is a particular problem at high switching rates such as at video switching rates for which the display panel is desirable.